Measuring devices, such as rulers and tape measures, have been used for a long time to measure the distance between two points. Typically one end of the measuring device is held or anchored at one point, and the position of a second point along the measuring device is noted.
When measuring the distance between two points that exceed arm's breadth, two people are typically involved in making the measurement with a tape measure. One person holds the end of the device at point one, and the second person extends the tape measure until it abuts the second point.
Devices have been invented and developed that allow one person to measure the distance between the device and, say, a point where two walls abut one another usually at right angles. In such cases, the device is placed at one end of a wall, and a laser beam is projected to the adjoining wall such that it reflects back along the same path and the distance is measured by noting the round-trip time and computing the distance based on light speed (e.g. d=c/2 t).
There are instances where one wants to measure the distance between two points that are high up on a wall, say, and would not be easily measured using a tape measure without having to use two people and two ladders, for example. In that case, using a laser-based device is a problem, too, because one would need the use of ladders and one would have to affix a reflecting target at the second point.
It would be very useful if one had a handheld device, like the laser “tape” measure, that could be pointed at one point, and then activated; and subsequently pointed at the second point, and activated; and it would quickly determine the distance between those two points.